styling: surface style_column exceptions in the column config instead of only in server logs

[paddymul]

Problem

When style_column raises, StylingAnalysis.style_columns (buckaroo/dataflow/styling_core.py:477-483) logs a warning and falls back to default_styling. The warning only goes to the Python logger (~/.buckaroo/logs/server.log for the server). Nothing in the column config sent to the frontend records that styling failed, so the browser UI, and callers that read initial_state off the websocket, can't tell a column that is plain obj on purpose from one whose styling code threw.

After #968 the failed column keeps its real header, so the table no longer looks broken, but the failure is now even easier to miss: one column quietly renders unstyled.

Suggested fix

Add an optional field to the column config, e.g. styling_error: str, set to f"{type(exc).__name__}: {exc}" in the except branch of style_columns. This needs:

• Python: add it as NotRequired[str] to BaseColumnConfig / NormalColumnConfig / MultiIndexColumnConfig (styling_core.py:134-158).
• TS: add styling_error?: string to BaseColumnConfig in packages/buckaroo-js-core/src/components/DFViewerParts/DFWhole.ts:176.
• Frontend: show it somewhere visible, e.g. a warning marker plus tooltip on the header. Make sure the field isn't passed through into the AG-Grid colDef.

The empty-col_meta pass during instantiation (len(col_meta) == 0 and len(cls.requires_summary) > 0) swallows its exception on purpose and should not set the field.

Context

Split out from the second suggestion in #966. The header fix is in #968.